A conventional inkjet-type printing device has a print head with rows of nozzles formed therein for ejecting ink droplets. This conventional printing device prints an image on paper by ejecting ink droplets from the rows of nozzles toward the paper while the print head is being conveyed in a main scanning direction orthogonal to the rows of nozzles.
This type of printing device prints in units of a strip-like area, referred to as a “band,” that has a width equivalent to the length of the nozzle rows. The printing device prints one band for each main scan of the print head. Thus, when printing an image over a region wider than a single band, such as a sheet of paper, the printing device repeatedly prints images in units of bands while shifting the position of the paper in the sub-scanning direction for each band unit.
Further, the number of gradations that the conventional printing device can render by ink drops ejected from the print head is less than the number of gradations (256-levels, for example) in the original image data representing the target image. Therefore, a halftone process is performed to produce print data with a fewer number of gradations, and the printing device ejects ink droplets from the print head based on this print data.
An inherent problem of this technology is that the printing device will sometimes produce a dark streak in the printed image along a seam between two adjacent bands printed in different main scans. Various techniques have been proposed to address this problem, including a technique for thinning pixels in regions of the print data produced from a halftone process that correspond to regions in the image in proximity to the seams (first technique), and a technique for reducing the quantity of ink ejected in such areas by reducing a droplet diameter rank for pixels in those areas (second technique). These techniques attempt to mitigate streaks that can be produced along the seams between pairs of neighboring bands printed in different main scans by processing the print data produced from the halftone process. The print data is processed to reduce the quantity of ink used when printing areas along the seams.